The invention disclosed herein is for assuring that the current flowing through an x-ray tube during an x-ray exposure corresponds with the current that has been selected by the x-ray technician or other operator.
As is well known, the current flowing between the target (anode) and the filament (cathode) of an x-ray tube depends mainly on the electron emissivity of the filament and to some extent on the kilovoltage (kV) that is applied to the anode. Emissivity is a function of filament temperature. In some systems, the voltage applied across the filament is varied to thereby change filament temperature, and, hence, emissivity. Such systems do not allow making closely consecutive x-ray exposures at different x-ray tube current levels (mA) because of the thermal lag of the filament; that is, the filament temperature does not change instantaneously with a change in applied voltage. Thus, it would be practically impossible to make one x-ray exposure at one tube current level and follow it with an exposure at another markedly different level in 30 milliseconds (ms), for example. It should be noted also that x-ray apparatus manufacturers calibrate the current control before the system is turned over to the user since there is a nonlinear relationship between filament temperature and tube current and anode voltage as well and it has taken time and skill to perform the calibration.
Closely consecutive x-ray exposures at markedly different tube currents or milliamperages (mA) can be achieved by using an x-ray tube that is equipped with a control grid. The user is typically required to actuate some x-ray tube current selection control which brings about a change in the negative bias voltage applied to the grid and, hence, the mA flowing through the tube. The usual bias voltage range is zero to minus 3,000 volts on the grid with respect to the filament. When grid bias voltage control is used, the x-ray tube filament current and filament temperature can be set at a fixed value since mA is controlled mainly by the grid bias voltage. Thus, the thermal lag problem is avoided and closely successive exposures at different currents can be made because the current response of the tube to bias voltage changes is substantially instantaneous.
When the filament temperature is held constant, the tube is operating in the emission limited mode. Tube mA can also be affected by space charge near the filament and by the anode-to-cathode kV at various grid bias voltages. Hence, x-ray apparatus manufacturers calibrate their tube current controls to apply a grid bias voltage that will yield the selected x-ray tube mA at many anode-to-cathode kilovoltages when the apparatus is installed for the user. The conventional calibration process involves iterative adjustment or trimming of a large number of potentiometers to obtain the various analog signals for creating the proper grid bias voltage in relation to selected x-ray tube mA and kV. Calibration of an analog signal control system requires a substantial amount of time which is obviously disadvantageous.
A further disadvantage is that the calibration applies only to the particular x-ray tube that is in the diagnostic x-ray apparatus at the time of calibration. Although x-ray tubes are manufactured to very close tolerances, tubes made on the same production line and having the same nominal ratings will have slightly different operating characteristics so there can be no universally applicable calibrating protocol. That is, tubes even of supposedly the same type can have a variable and rather unpredictable relationship between selected mA, grid bias voltage and applied kV. It should be evident that if an x-ray tube has to be replaced with a comparable tube in any diagnostic x-ray apparatus, the laborious calibrating procedure must be repeated on the user's premises where calibration involves setting the levels of many analog signals in accordance with the prior art method mentioned above.
Hybrid digital subtraction angiography (HDSA) is an x-ray diagnostic procedure that requires an accurate and reproducible relationship between the bias voltage that is applied to the control grid and the electron emission current in an x-ray tube. In the HDSA procedure an alternating series of low kV-high mA and high kV-low mA x-ray exposure pairs are made of a region in a body that contains the blood vessel of interest. A high x-ray energy exposure in a pair is made one or two television frame times after the low energy exposure, for example, 30 ms or 60 ms apart to reduce the likelihood of body movement between exposures. The first sequence of exposures are made before an x-ray opaque medium such as an iodinated compound, reaches the region of interest. The data representative of the x-ray images are stored. The exposure sequence continues over an interval during which an injected opaque medium reaches the vessel of interest, increases to maximum concentration and decreases to low or zero concentration. All the image data are stored. In one of the hybrid data processing procedures, the low x-ray energy exposures and high energy exposures are summed and weighted and the summations are combined to bring about cancellation of soft tissue in the region of interest, and let data representative of the image of the opaque medium filled blood vessels remain. More information on HDSA can be found in Keyes et al, application Ser. No. 371,683 filed Apr. 26, 1982, now U.S. Pat. No. 4,482,918 dated Nov. 13, 1984 which is assigned to the assignee of this application.
The patent just cited illustrates a case where x-ray exposures are made with low kV on the anode of the x-ray tube in combination with high mA flowing through the tube (called low energy exposures) alternating with exposures made with higher kV on the anode and the lower mA (called high energy exposures). For digital subtraction angiography it is especially important to obtain and maintain tube currents during high energy exposures that correlate with tube currents and anode kV's used for the low energy exposures. One reason is that it is desirable to have substantially the same x-ray dosage or milliroentgens for the low energy exposures as for the high energy exposures.